The purpose of this study was to test the hypothesis that expired carbon dioxide from each lung accurately reflects the blood flow to that lung. To do this, an experimental model was created whereby pulmonary blood flow could be determined directly using electromagnetic blood flow determinations for the right and left pulmonary arteries individually. Simultaneous determinations of the expired CO2 from each lung were made using a double lumen endotracheal tube which permitted separate but simultaneous ventilation of each lung. The expired gases were analyzed continuously from a sampling port in each lumen to a Chemetron Mass Spec II spectrophotometer.